1. Field of the Invention
The present invention relates to a photosensor employed for monitoring a scanning position of a light beam, and an image scanner in which an image is read or recorded under scanning control using the photosensor.
2. Description of Prior Arts
In a process scanner of flat bed type and the like, it is necessary to periodically deflect a light beam along a main scanning direction for reading or recording an image. A rotating or oscillating light deflector, such as a polygon mirror, a galvano mirror, a hologram disk or the like, is employed for such periodic deflection.
However, a locus of a light beam which is deflected by such a light deflector does not necessarily define a complete straight line on a scanned plane, but is curved in a configuration specific to the light deflector. For example, in the case of a polygon mirror, it is known that such curvature is caused by surface inclination of mirror surfaces, and such curvature must be corrected so that the locus of the light beam defines a straight line. Similarly, wobbling correction is required in the case of a galvano mirror, and scanbow correction in the case of a hologram disk.
As a technique for performing such correction, U.S. Pat. No. 4,661,699 discloses an apparatus for detecting a reference light beam which is deflected together with a main light beam by a photosensor, and compensating for projection deviation of the main light beam to a subscanning direction on the basis of a photoelectric conversion output of this photosensor. This photosensor is formed by pasting a glass member provided with a stripe light shielding pattern on a major surface of a long photoelectric conversion element, the glass member is called a grating glass member. Unit patterns forming the stripes have tapered portions, so that an output waveform from the photoelectric conversion element is changed in response to the value of projection deviation of the light beam in the subscanning direction. The scanning positions of the main light beam and the reference light beam in the subscanning direction are changed in response to the output waveform, thereby correcting the projection deviation.
On the other hand, Japanese Patent Laying-Open Gazette No. 63-278020 discloses a technique of arranging two independent photosensors in parallel with each other for receiving a reference light beam, and correcting projection deviation of a light beam on the basis of photoelectric conversion signals thereof.
However, in the photosensor disclosed in U.S. Pat. No. 4,661,699, it is not easy to manufacture the grating glass member employed therefor, and the manufacturing cost is considerably increased as the result. Such a tendency is remarkable particularly since the specific patterns having tapered portions are required.
In the technique described in Japanese Patent Laying-Open Gazette No. 63-278020, on the other hand, the two independent photosensors must be rendered adjacent to each other with a uniform space in the subscanning direction, and hence it is difficult to adjust relative positions in integration of these photosensors.